The present invention relates to sensors and more particularly to simple sensors having piezoelectric material in the transducer.
Considerable effort has gone into the development of mechanisms which function to measure or locate the level of a material in a container. Typical of such devices is the apparatus taught by Mongan in U.S. Pat. No. 3,010,318 entitled Method and Apparatus for Measuring Liquid Levels. The essence of the Mongan device includes a transmitter and a receiver with an interconnecting waveguide running therebetween such that ultrasonic waves produced at the transmitter are passed through the waveguide to the receiver. In operation, the waveguide is positioned to pass through the liquid vapor interface and since the impedance of the waveguide will vary depending upon the amount of surface in contact with the liquid thereby affecting the amount of transmitted power which reaches the receiver, with suitable calibration of the device of the signals arriving at the receiver can be converted to liquid level position. This relatively large device uses multiple transducers, is cumbersome and expensive for many applications.
In the Fetal Heart Transducer described by J. R. Richards in U.S. Pat. No. 3,379,901, an apparatus comprising two circular transducers mounted concentrically is described. One transducer acts as a transmitter and the other as a receiver in the transmission of sonic energy through liquids to investigate a fetal heart by studying the doppler shift in the reflected waves. The Richards' apparatus comprises two concentric piezoelectric crystals which are located in a disc assemblage and are carefully isolated physically and electrically from each other.
U.S. Pat. No. 3,625,058 to Endress et al liquid is present. Two parallel vibrator rods are induced to vibrate by a piezoelectric transducer and a second transducer senses the rod motion and provides an output signal which is proportional to the amplitude of such motion. When the mechanism is immersed in a liquid the vibrational frequency changes and is sensed in the receiving transducer. Another transducer containing sensing apparatus is disclosed by Samuel et al who describe a transducer for sensing the presence of a liquid in U.S. Pat. No. 3,825,025. The basic apparatus comprises two piezoelectric elements which are spaced apart from one another and means for inducing ultrasonic vibrations in one of the elements. The coupling between the two piezoelectric elements is a function of the level of liquid which is present in the physical space left between the two transducers and which is oriented in a vertical direction so that with suitable calibration the system can be used to determine fluid level.
A simple rugged device which can be built inexpensively and perform with great reliability such as that demanded for liquid level sensing in automobile applications is needed.